The cortical processing of sensory information depends critically on the orchestrated activity of interconnected excitatory and inhibitory neurons. Understanding the structure of these excitatory and inhibitory synaptic circuits is key for comprehending how information processing is achieved in the cortex, but the progress has been limited previously by technical difficulties in bridging connectivity and function. The recent development of molecular and genetic tools in the mouse makes it an attractive model for systematically dissecting synaptic circuitry underlying cortical functions. Taking advantage of these tools, we will be able to integrate multiple approaches to address synaptic circuitry mechanisms for the fundamental receptive field properties of mouse primary visual cortical neurons. In this project, we will first reveal the spatiotemporal interplays of visually evoked excitatory and inhibitory synaptic inputs to excitatory neurons with in vivo whole-cell voltage-clamp recordings. Specifically, we will determine the synaptic mechanisms underlying two fundamental visual processing properties of cortical excitatory neurons, the directional selectivity and the contrast invariance of orientation selectivity. We will then dissect the functional contribution of excitatory inputs from different origins with optogenetic methods. By silencing the cortex with optogenetic activation of a specific group of inhibitory neurons, we will determine the respective contribution of thalamocortical and intracortical input to orientation selectivity of layer 4 excitatory neurons. Finally, by developing two-photon imaging guided whole-cell voltage-clamp recording techniques in transgenic mouse models, we will determine the synaptic mechanisms for the weak orientation tuning exhibited by pavalbumin-positive cortical inhibitory neurons. These proposed studies will potentially provide important new insights into how functional cortical synaptic circuits are organized and how cortical processing and sensory perception may go awry under neurological disease conditions which result in disrupted excitation-inhibition balance. PUBLIC HEALTH RELEVANCE: Understanding the organization of synaptic circuits that determines the normal functional properties of individual cortical neurons is necessary for identifying circuit components that may go awry in psychiatric and neurological disorders. In this project, we propose to unravel the excitatory and inhibitory synaptic circuitry mechanisms for fundamental visual processing functions in the mouse visual cortex by integrating several innovative in vivo approaches. The proposed studies will be able to generate new levels of information for our understanding of the physiology and pathology of the visual cortex, in particular of how changes in the balance of excitatory and inhibitory signaling as implicated in several neurological diseases can lead to abnormal perceptual functions.